The Automatic Identification System (AIS) is an automated signaling system used by ships to identify and avoid collisions with other ships.
An AIS signal contains a preamble selected from one of a number of fixed preambles, a ship's identity, position, course, and speed. AIS systems use Gaussian Minimum Shift Keying (GMSK) modulation and Self Organizing Time Division Multiple Access (SOTDMA) regions. Inside a SOTDMA region, a user broadcasts information in dedicated time slots, which prevents AIS signals from interfering with each other. However, AIS signals originating outside a SOTDMA region may interfere, or collide, with an AIS signal originating within the SOTDMA region. When this happens, demodulation in co-channel interference becomes necessary.
AIS signals employ GMSK demodulation. In GMSK, an information bit, either a −1 or a 1, is transmitted at each baud time, nT, where n is an integer, T is a constant bit duration or baud interval, and where t is time. A GMSK signal has the following form:s(t)=eiφ0eiφ(t)e2πƒct,
where e raised to a value is the exponential function, i is the square root of −1, φ0 is carrier phase, φ(t) is the phase function of s(t), t is time, π is a constant, and ƒc is carrier frequency.
Articles entitled “A simple matched filter approach to AIS demodulation in co-channel interference,” Proc. of the Eleventh LASTED Int'l Conf on Signal and Image Processing, pp. 71-76, August 2009, and, “A comparison of two methods for demodulating a target AIS signal through a collision with an interfering AIS signal,” Proc. of SPIE Mathematics for Signal and Information Processing, vol. 7444, pp. 0W1-0W12, Aug. 2-5, 2009, disclose demodulating an AIS signal when the carrier frequency, carrier phase, and time alignment offset of the signals is known. In the present invention, it is not assumed that these parameters are known.
An article entitled “GMSK Co-Channel Demodulation,” Proc. of SPIE Mathematics for Signal and Information Processing, vol. 7444, pp. 0V1-0V11, Aug. 2-5, 2009, discloses demodulation of an AIS signal without first removing carrier phase as does the present invention.
An article entitled “Inter-channel interference rejection for maritime AIS system,” The 5th Int. Conf. on Intelligent Transportation Systems Telecomm., Jun. 27-29, 2005, discloses the use of a Tukey window and a Chebyshev lowpass filter to avoid spreading of the spectrum.
An article entitled “Joint Demodulation of Low-Entropy Narrowband Cochannel Signals,” 2007 NRL Review, pp. 207-210, 2007, discloses the use of field-based maximum a posteriori (MAP) joint detection algorithm that uses available a priori information to aid in jointly making a decision of what values are sent. The method tries all permutations and selects the one that maximizes the a posteriori probability. The authors indicate that this method is impractical for all but the shortest of messages, unlike the present invention.
An article entitled “AIS Information Decoding and Fuzzy Fusion Processing with Marine Radar,” IEEE, pp. 1-5, 2008, discloses decoding AIS signals using multi-factor fuzzy integration decision-making, utilizing information of the target's distance, bearing, velocity, and course, unlike the present invention.
An article entitled “A comparison of two methods for demodulating a target AIS signal through a collision with an interfering AIS signal,” Mathematics for Signal and Information Processing, Proc. of SPIE, Vol. 7444, pp. 74440W-1-74440W-12, 2009, compares the Laurent-Kaleh demodulation method to a demodulation method that uses filters matched to tri-bit states and phases of GMSK signals (i.e., tri-bit demodulation).
U.S. Pat. No. 7,805,146, entitled “CELL PHONE PDA/GPS COMMUNICATION NETWORK WITH AIS,” discloses a method of interfacing a cellular phone to an AIS signal. U.S. Pat. No. 7,805,146 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 7,809,370, entitled “SPACE BASED MONITORING OF GLOBAL MARITIME SHIPPING USING AUTOMATIC IDENTIFICATION SYSTEM,” discloses a device for receiving an AIS signal by a space vehicle and determining the accuracy of the transmitter's location, including determining its accuracy as compared to the location of a space vehicle. U.S. Pat. No. 7,809,370 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. Appl. No. 20080086267, entitled “SPACE BASED NETWORK FOR DETECTION AND MONITORING OF GLOBAL MARITIME SHIPPING USING AUTOMATIC IDENTIFICATION SYSTEM,” discloses a device where a plurality of satellites receive an AIS signal, where the satellites are distributed in two or more orbital planes located at substantially common altitude and having a substantially common inclination. U.S. Pat. Appl. No. 20080086267 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. Appl. No. 20080088485, entitled “SPACE BASED MONITORING OF GLOBAL MARITIME SHIPPING USING AUTOMATIC IDENTIFICATION SYSTEM,” discloses a device where a space vehicle receives an AIS signal and determines the accuracy of the transmitter's position. U.S. Pat. Appl. No. 20080088485 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. Appl. No. 20080304597, entitled “SYSTEM AND METHOD FOR DECODING AUTOMATIC IDENTIFICATION SYSTEM SIGNALS,” discloses a device and method of receiving a plurality of AIS signals, pre-processing the signals to produce digital data, and identifying one or more candidate AIS signals based on Doppler offsets and time estimates associated with the digital data, determining corresponding Doppler offset estimates of the candidates, and decoding the candidate signals. U.S. Pat. Appl. No. 20080304597 is hereby incorporated by reference into the specification of the present invention.
U.S. Pat. Appl. No. 20090161797, entitled “SATELLITE DETECTION OF AUTOMATIC IDENTIFICATION SYSTEM SIGNALS,” discloses a device for receiving a plurality of AIS signals, producing digital date there from, identifying one or more candidate AIS signals based on Doppler offsets associated with the digital data, determining corresponding Doppler offset estimates and time estimates of the candidate signals, and decoding the signals. U.S. Pat. Appl. No. 20090161797 is hereby incorporated by reference into the specification of the present invention.
There is a need for a device for and method of demodulating an AIS signal when carrier frequency, carrier phase, and time alignment offset are unknown. The present invention is such a device and method.